Kavango Sand and Water Chapter 6 2003.Pdf
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Chapter6.qxd 5/19/03 3:53 PM Page 56 sandVIandwater CHAPTER SIX Chapter6.qxd 5/19/03 3:53 PM Page 58 RESOURCES | Natural wealth sandVIandwater 1 Kavango looks the way it does because about less than /4 of a percent of the total average it was moulded by various geological and historical volume of water that enters Botswana at Mohembo, and events, many of which were described in Chapters 2 even during years and months with low flows, the Figure 26- and 3. The region’s plants and animals were likewise amounts removed from the river in Kavango are Underground water is usually shaped by evolutionary processes of long ago, but the relatively small. However, several new and large found at much shallower way in which these organisms live today is also a more irrigation schemes are being developed and planned depths in eastern than direct consequence of three fundamental inputs: (see page 99) and, once implemented, these would raise western Kavango. The scale to energy from the sun, water provided by rain, the river the total amount of water extracted from the river to the map shows the average or from underground, and the soils in supplying a about 136 Mm3 per year, or about 1.4% of all water that depth at which borehole medium in which plants can grow and draw nutrients leaves Namibia at Mohembo. This proportion still water is pumped from below and water. These three ingredients -- alone and in appears small but the effects of removing these amounts the surface. The red lines combination -- determine the nature and abundance of of water on the health of the river, especially the indicate the approximate living organisms in the region. Okavango Delta in Botswana, have not been assessed. boundaries of four zones in Solar energy is obviously abundantly available in A good deal of irrigation water would be pumped at the which underground water providing energy for plant growth and heat to warm start of the growing season in early summer when the flows in different directions, 3 the air and ground. By contrast, water is a severe volume of water in the river is at its lowest. Moreover, as shown by the arrows. problem, especially for anything that lives any the proportion of water extracted during the driest distance from the river. Rainfall is highly seasonal and months in years when flows are unusually low could be unpredictable (see Chapter 4), and the growth of crops substantial. No regulations or quotas control the use of and natural vegetation is often limited by a shortage of river water in Angola, Namibia or Botswana. rainwater. Underground water is beyond the reach of The supply of water is one thing, but having access to most roots, and it is only through the recent use of uncontaminated water is also important for reasons of boreholes and pumps that groundwater can be used for avoiding diseases caused by bacteria and other parasites Figure 27- people and livestock, as described below in this in dirty water (see page 85). Water sources are Most boreholes provide chapter. Soils, too, are a severe problem because they commonly divided into those that are unsafe moderate supplies of water are so infertile and hold little water in most areas. (potentially contaminated water from the river or hand- sufficient for the small This chapter begins with accounts of underground dug wells) and those that are safe (underground or villages. The map shows water and soils as two of the fundamental inputs on borehole and treated river water). In 2000, average yields from boreholes which life in Kavango depends. Then follows approximately 52% of all households used so-called measured in cubic metres of information on the region's natural vegetation and the ‘safe’ sources, a slight improvement from 46% in 1991. water pumped to the surface 4 major impact that repeated bush fires have on the The great majority of homes that use unsafe water are per hour. structure and composition of woodlands. A variety of in rural areas. Thus in 2000, only 40% of the rural aspects concerning animal life in Kavango are population had access to safe water compared to 82% of described with a special focus on the abundance and people in Rundu.2 distribution of larger mammals. A final section Most underground water is provided through considers the region's conservation areas and the use boreholes drilled and supplied with pumps by the that tourism makes of these natural parks. Department of Water Affairs, although a few wealthier farmers have also installed their own boreholes and Figure 28- pumps. Borehole pumps generally draw water using The supply of water Underground water in the People and livestock in Kavango get their water from diesel, wind or electrical (often solar powered) energy, region is usually of good two sources: the river and from boreholes that pump whereas water in hand-dug wells is usually winched to quality, as shown by the values water from under the ground. Various aspects the surface by hand. The main effect of having access to of total dissolved solids (TDS) in concerning the flow of water in the Okavango River are groundwater has been to enable people to settle in milligrams per litre. The purest described in Chapter 5, but what is important here in the inland areas where they would otherwise have been water has low TDS values, and context of water supply is that only a small fraction of unable to live (see page 114). However, underground humans can drink water with the water is used. The total amount of river water now water has also been of value in places close to the river, values of less than 2,000. Water used in Kavango each year is about 22 million cubic especially where it is used to provide safe water to bulk with values above 2,600 should metres (Mm3), of which approximately 74% supplies water supply schemes for schools, hospitals and the be avoided, while even live- agricultural irrigation schemes, 15% is used by rural public at such places as Nkurenkuru, Kahenge, stock should not drink water people for their livestock and domestic needs, and 11% Tondoro, Rupara, Bunya, Mupini, Kayengona, having TDS values above 5,000.5 is for urban use in Rundu.1 The 22 Mm3 amounts to Shambyu and Nyangana. 58 59 Chapter6.qxd 5/19/03 3:53 PM Page 60 RESOURCES | Natural wealth sandVIandwater Most underground water lies in bodies of water -- zones of movement of underground water (Figure 26). Figure 29- known as aquifers --in the Kalahari sediments (see page The first is in the north-west where water permeates Wind-blown sands (arenosols) cover much of Kavango and are generally not suited to 24). The water is trapped under pressure in tiny spaces south-eastwards from Angola. A second zone is in the crops. Smaller patches of somewhat more fertile soils (anthrosols, fluvisols and between the grains of sand and other sediments. The centre of Kavango, and here water flows radiate out calcisols) are found along the Okavango River, the omurambas and inter-dune valleys.6 aquifers lie at various depths (Figure 26), those in the from the vicinity of Ncamagoro. Flows also spread out eastern half of the region being at the shallowest levels in a third zone in the south-east, while water flows of 10 to 30 metres below the surface. Water in western north into Kavango from the Grootfontein area in the Kavango is usually found below 60 metres, and water in fourth zone. some places has only been found at depths of as much Shallow aquifers in the east are recharged much as 350 metres. The main effect of these different depths more rapidly by good falls of local rain than the deeper is that deeper water is much more expensive to find and aquifers into which water seeps over much longer pump. Other than aquifers in the Kalahari sediments, the periods. Water is also relatively easy to find in the only other underground water bodies are in Damara eastern shallower aquifers, and yields of 5--10 cubic Sequence rocks of the Nosib Group in the area of metres per hour can generally be obtained (Figure 27). Andara and Popa Falls (see page 24). These are adequate to supply the household and The shallow aquifers along the river often consist of livestock needs of small villages. There is, however, a shallow bodies of water trapped in old channels of the good deal of variation in yields, even between Okavango River. Most of the water seeping into those ancient deep channels probably flows in from the south. Indeed, these movements of underground water Water is often transported over from the south mean that the Okavango River gains substantial water from shallow aquifers, rather than the river distances, and water recharging aquifers. Flows of underground water people in Kavango into the river valley occur in three of four separate use many methods of carrying water to their homes. Soils in the bottoms of omuramba valleys are more clayey (and thus dark) than the pale coloured sands higher up and away from the valley. This is the Ndonga Omuramba. boreholes close to each other. Most differences are due to three than 1,000 milligrams per litre (Figure 28). Water unsuited for factors: (a) differences in the permeability of the sediments in use has been found in only a few areas, in many of which the which the water is trapped, (b) the thickness of the aquifer (and water was probably contaminated by animal waste, especially the depth to which the borehole penetrates the aquifer), and (c) where cattle troughs or kraals were close to wells drawing water the diameter of the borehole well.